1. Field of the Invention
The present invention relates to a photographing light quantity controller for an endoscope, which is used to control the quantity of illuminating light when a photograph is to be taken through the endoscope.
Endoscopes are generally designed to be capable not only of observing the inside of a hollow organ in the patient's body but also of taking a photograph of it. The exposure time for the photography has heretofore been controlled by integrating the output of a light-receiving element which receives the reflected light from an object that is illuminated with a light source device, and closing a mechanical shutter, which is provided in an illuminating light path inside the light source device, when the integral output voltage reaches a set voltage.
However, such a mechanical shutter takes a time from the instant it receives a signal for closing until it has been completely closed (about 0.05 seconds in general), and the exposure therefore becomes correspondingly excessive. The exposure time .DELTA.T that corresponds to an excess of exposure is constant independent of the length of the overall exposure time on each particular occasion. Accordingly, when the overall exposure time T is relatively short, that is, when the object is relatively bright as in the case of close-up photography, the effect of .DELTA.T becomes significant, resulting in a high degree of over-exposure.
2. Description of the Background and Relevant Materials
To reduce the degree of over-exposure, a method has heretofore been employed in which the rise of a signal representative of an integral state value which is obtained by integrating the output of a light-receiving element is detected differentially. When the differential output value is greater than a reference value, the brightness of a light source is lowered to an observational state level to increase the overall exposure time T, thereby reducing the effect of .DELTA.T.
In such a device, the lower the reference value, the wider the range within which the degree of over-exposure can be reduced. However, as the reference value lowers, the exposure time lengthens, causing the problem of blur. The reference value must therefore be set at a relatively high level. In consequence, considerable over-exposure still occurs in an intermediate distance range within which the differential output is lower than the reference value.